In transport vehicles such as automobiles, railcars, and aircrafts, spot welding or rivet connection has been used for coupling metal members together. However, in recent years, friction stir spot welding, in which metal members are joined together by utilizing frictional heat, has been drawing attention.
In friction stir spot welding, a columnar rotary tool (welding tool) is used. The rotary tool is configured to be movable to forward and backward relative to workpieces. The rotary tool is, while rotating at a high speed, pushed (press-fitted) into the workpieces (metal members). The metal members become softened at their portion into which the rotary tool is press-fitted. By stirring the softened metal members, the workpieces are joined together.
It is known that in order to realize joining with higher quality by such friction stir spot welding, it is necessary to suitably control the displacement of the rotary tool when the rotary tool makes forward/backward movement.
For example, Patent Literature 1 discloses a technique in which: the amount of insertion of a friction stir welding tool (rotary tool) inserted into a layered portion (formed by layering a plurality of members together) is measured; and the displacement of the friction stir welding tool is controlled such that the actual measurement value coincides with a target value. Patent Literature 2 discloses control that is performed in double-acting friction stir spot welding, the control being performed in the following manner: in a case where a clamp member is provided on the outside of a shoulder member, calculate a distance between the tip of the clamp member and the tip of a rotary tool (a pin member or shoulder member), thereby adjusting the press-fit depth of the rotary tool.